1. Field of the Invention
The present invention is directed to a system for transmitting digital signals in a radio subscriber line network and more particularly to a system for transmitting digital signals in a broadband RLL (radio in the local loop) subscriber line network.
2. Description of the Prior Art
A radio subscriber line network is a system of radio cells that respectively contain a stationary base station around which the networks terminations (NT""s) of the radio subscribers are located in a radius of, for example, 1 km. A system is typically area-covering wherein the base station is usually placed as centrally in the cell as possible and the network terminations are more or less uniformly distributed in the radius. FIG. 1 shows such a radio cell 10, in which the base station 12 is referenced BS and the radio subscribers (or their network terminations) are referenced NT. Since the radio field attenuation in a radio system 10 quadratically increases with the distance, the radio field strength at the cell edge is significantly lower than in the inside of the cell. In FIG. 1, this attenuation is illustrated with concentric circles around the base station 12. Additional attenuations, which are caused, for example, by rain, and which are likewise distance-dependent, can additionally greatly reduce the reception power available at the network terminations NT at the cell edge.
In such a radio system 10, which fundamentally represents a point-to-multipoint system (the transmitter of the base station 12 can reach the receivers of many subscribers NT), the signal transmission from the base station 12 downstream to the radio subscribers NT can proceed in time-division multiplex (TDM) in a 155 Mbit/s bit stream, and the signal transmission from the radio subscribers NT upstream to the base station 12 can proceed in a TDMA (time division multiple access) access method. in radio channels having a correspondingly higher or lower number of bits per message signal element, whereby the multiply transmitted bits are in turn respectively combined to a single bit at the reception side.
With such a power scaling (also, advantageously, simple to implement in an already existing TDM/TDMA system) in accord wherewith a multiple transmission of bits potentially occurs dependent on the distance between base station and radio subscriber and a radio channel of exactly the required number of bits per symbol is assigned therefor to the respectively already existing connections between base station and radio subscribers, the invention advantageously enables an optimum power utilization within the radio cell, whereby the overall transmission power of the base station can be correspondingly reduced or (given unaltered overall transmission power) the power that has been gained can be used for increasing the cell radius.
The bit number per message signal element (symbol), i.e. the repetition factor with which a bit is multiply transmitted in immediate succession, can be an arbitrary whole number n, so that a correspondingly fine, potentially adaptive matching to the respective radio channel attenuation is possible. The increase of the bit number per message signal element, or expressed in other words, the multiple, immediately succeeding transmission of a bit in the framework of the aggregate bit rate of the system given combination to a correspondingly xe2x80x9clongxe2x80x9d bit (or a single symbol) has the following effect:
Each doubling of the bit duration by 2-fold (and, further, 4-fold, 8-fold . . . ) transmission of the same bit doubles the part of the overall transmission power devolving onto the bit (or the symbol corresponding to it), i.e. the proportional signal power S becomes respectively 3 dB greater. In the time domain, this has the direct effect of 2-fold (and, further, 4-fold, 8-fold . . . ) energy per individual symbol.
Each such doubling of the bit number per symbol simultaneously halves the bandwidth of the signal; the transmission of the overall transmission power Ps within half the respective bandwidth increases the power/Hz by respectively 3 dB.
By halving the bandwidth, the noise power R per symbol is simultaneously reduced by respectively 3 dB given appropriate low-pass filtering.
Overall, the signal/noise ratio S/N for n=2, 3, 4, 5, 6, . . . bits/symbol is thus improved by 9 dB or 14 dB or 18 dB or 21 dB or 23 dB, etc., i.e. the signal/noise ratio S/N is raised by 9 dB given every doubling of the bit duration.
Due to the concentration of the entire transmission power, resulting from the multiple transmission of bits, onto a correspondingly smaller frequency band, the channel capacity is optimally utilized for radio subscribers (network terminations) at a greater distance from the base station. In systems of limited power, no power or channel capacity is xe2x80x9cgiven awayxe2x80x9d. Since the influence of multipath effects is greatly diminished due to the lengthened bit or symbol duration, the digital signals, in an expedient development of the invention, can also be transmitted on radio links with inadequate signal/noise ratio with a correspondingly higher bit number per message signal element (symbol) to radio subscribers lying closer to the base station.